The present invention relates generally to the field of drop marking, including ink jet printing, and more particularly to a device to control the height of characters produced thereby.
The present invention may be applied to charge deflection printers of a type known in the art. Such printers employ a piezoelectric element to break a constant stream of marking fluid, such as ink, into uniform drops. The drops are then electrically charged by charging electrodes. The charged drops travel through an electric field created by deflection electrodes. The influence of the deflection field on the drops alters their flight path.
Most drop marking devices employ a pressurized distribution system for conveying marking fluid from a supply cheer to the nozzle and a collection reservoir to capture drops not intended to mark a target. The operation of most drop marking devices is controlled by a system microprocessor.
Use of a control system to maintain a constant ink flow rate in ink jet systems is known in the art. Examples of such systems are shown in U.S. Pat. No. 4,555,712 ("the Arway patent") and U.S. Pat. No. 4,827,280 ("the Stamer patent"). These control systems measure the flow rate of the marking fluid in the system and alter variables such as the amount of solvent contained in the ink, the magnitude of pressure applied to force ink through the system or ink temperature to keep the flow rate at a predetermined value.
Even with these control systems, a problem has been the inability to accurately control the height of printed characters. Height of printed characters has been found to vary due to a number of environmental factors, such as operating temperature of the printer and ink solvent evaporation. The prior art control systems lack the ability to fully compensate for variations in character height resulting from these factors.
Some attempts have been made to maintain constant character height for charge deflection printers by making adjustments when the printer is initially set up or during maintenance. One such scheme is disclosed in U.S. Pat. No. 4,847,631 ("the Naruse patent"). There, a user sets a predetermined character height when the print head is replaced. The Naruse patent discloses a relationship between the stream velocity of marking fluid and character height. Naruse, however, does not compensate for changes in character height over time due to the external factors previously described, principally changes in operating temperature.
A marking device capable of monitoring and adjusting the character height on an ongoing basis is desirable as it enhances marking quality.
Accordingly, it is an object of the present invention to provide a control system capable of automatically adjusting the character height for use with drop markers such as ink jet printers.
It is a further object of the invention to provide such a control system which has the capability of minimizing the influence on character height of external factors such as variations in printer operating temperature.
It is still another object of invention to provide such a control system capable of operation without regard to physical characteristics of the marking fluid used in the marking device.
It is an additional object of the invention to provide such a control system that is durable in construction, efficient in operation, and inexpensive to implement.
These objects, as well as others, will become apparent to those skilled in the art from the detailed description of the invention provided below.